A common component used in electronic circuits is an amplifier. Differential amplifiers, for example, are widely used in various types of analog integrated circuits (ICs), such as operational amplifiers, voltage comparators, voltage regulators, video amplifiers, and modulators and demodulators. Likewise, differential amplifiers also find use in emitter-coupled digital logic (ECL) gates and as the first stage of operational amplifiers and other integrated circuits (ICs).
Amplifiers may be configured to have open loop or closed loop arrangements. These alternative arrangements have corresponding advantages and drawbacks. For example, in a closed loop arrangement, also known in the art as a negative feedback arrangement, an inverted portion of the output signal of an amplifier is combined with its input signal. It is well-known that this arrangement provides relatively good gain tolerance. Thus, the gain of the amplifier remains substantially constant in response to changes in characteristics of the components of the amplifier. For example, the gain of an amplifier employed in a closed loop arrangement remains substantially unaffected by mismatches in the transistors that form the amplifier. However, the frequency response of a negative feedback amplifier is limited. This may be an issue for applications in which the amplifier is used to amplify high frequency communication signals. For input signals above a certain frequency, the amplifier may become unstable. As the frequency of an input signal increases, the phase of the output signal of the amplifier changes. At a certain frequency, the phase of the output signal may deviate 180 degrees from the phase of the input signal. At this frequency, the negative feedback signal becomes a positive feedback signal and may cause oscillation.
In contrast, an amplifier employed in an open loop configuration has a very wide bandwidth response. Open loop amplifiers accommodate signals over a wide frequency spectrum, without the disadvantages associated with amplifiers employed in closed loop configuration, as mentioned above. Thus, when operating at higher frequencies, an open loop amplifier may be more desirable than a closed loop amplifier. However, one drawback of open loop amplifiers is the difficulty of maintaining a relative constant gain in situations where there are mismatches between the transistors that form the amplifier. Some open loop amplifier designs, like the one illustrated in FIG. 2, provide a relatively constant gain.
FIG. 2 illustrates a prior art differential amplifier 10, including input transistors 12 and 46, configured to receive a differential input signal, such as an input voltage signal, to be amplified. Source 16 of input transistor 12 and source 42 of input transistor 46 are coupled together and to a current source 48. The current source may be realized in a variety of ways, such as using a MOSFET transistor that operates in its saturation region, or a BIPOLAR transistor that operates in its active region. Source 26 of a load transistor 24 is coupled to drain 18 of input transistor 12. Similarly, source 34 of a load transistor 36 is coupled to drain 40 of input transistor 46. Gate 22 and drain 20 of load transistor 24 are coupled together and to a direct current (DC) power supply 28 that provides a substantially DC voltage signal, V.sub.DD. Similarly, gate 32 and drain 30 of load transistor 36 are coupled to each other and to DC power supply 28. Load transistors 24 and 36 operate as active loads. Active loads are well-known, and described in Analog Integrated Circuits, by Sidney Socolof (Prentice Hall, 1985), incorporated herein by reference. Typically, a differential amplifier, such as amplifier 10 illustrated in FIG. 2, in response to an alternating current (AC) input signal, generates an AC output current signal in each one of its branches, such as drains 18 and 40 in this example. An active load, such as load transistor 24 or 36, in this example, transforms the AC output current signal into an output voltage signal across terminals 38 illustrated in FIG. 2. As will be explained in more detail, hereinafter, the conductance of an active load, at least in part, contributes to the amount of the gain of the amplifier.
However, a drawback of an amplifier employed in an open loop configuration, like the one illustrated in FIG. 2, is a relatively lower power supply rejection ratio (PSRR), compared to amplifiers employed in closed loop configurations. DC power supplies, providing a DC power signal to such amplifiers, may sometimes generate AC ripple or noise signals that may influence the output signal of an amplifier. PSRR, as explained in more detail hereinafter, is a measure of the extent to which the AC noise signals of a power supply affect the operation of the amplifier.
For amplifiers employed in closed loop configurations, the PSRR is usually acceptably high. However, for amplifiers employed in open loop configuration, like the one illustrated in FIG. 2, noise signals generated in DC power supply 28, may influence the output voltage of the amplifier due to a feedthrough of supply voltage fluctuations into the signal path of the amplifier. Supply voltage noise or fluctuations may be caused, for example, by an AC ripple or noise signal present at the output terminal of the DC power supply. Typically, a DC power supply uses a rectifier to convert an AC line voltage signal to a DC voltage signal followed by a filter circuit. AC ripple or noise signal may propagate to the output of the power supply. Other types of noise and interferences in the AC line voltage may also cause supply voltage fluctuations.
The power supply rejection ratio, PSRR, in this context is defined as the ratio of change in the input offset voltage resulting from a change in the supply voltage to the change in the supply voltage, and is usually expressed in decibels (dB). For input signals with relatively small voltage amplitudes, a relatively high PSRR is desirable. Otherwise the signal-to-noise ratio of the amplifier would become unacceptably low.
Thus, a need exists for an open loop amplifier with a substantially high power supply rejection ratio, that provides a substantially constant gain over a wide bandwidth and also provides an output voltage signal, in which noise signals generated by the DC power supply that drives the amplifier are substantially attenuated.